Nickel-hydrogen cells, as shown in FIG. 1, have been used in space applications for many years. The main advantages of nickel-hydrogen cells are high specific energy, long cycle life, long functional lifetime, and fairly robust chemistry. The long cycle life has been used to advantage in low earth orbit (LEO) applications where a satellite will orbit the earth every 90 minutes. Roughly 35 minutes of this orbit is spent in the earth's shadow, during which time the satellite must use the batteries for power. The other 55 minutes of the cycle is spent in sunlight, during which time the satellite is powered by solar panels that must also recharge the batteries. In these applications, up to 50000 charge-discharge cycles are common for nickel-hydrogen cells.
Nickel-hydrogen cells are essentially a hybrid of standard cells and fuel cells. The positive electrode is nickel (NiOOH), and the negative electrode is hydrogen that diffuses into a porous platinum electrode. The platinum only serves to provide a reaction site for the hydrogen gas, and does not participate in the reaction. The entire cell consists of a large sealed pressure vessel that contains the hydrogen gas and the electrode stack. The electrode stack alternates pineapple-slice shaped disks of nickel electrode, separator (containing the electrolyte, KOH), platinum electrode, and a hydrogen diffusion screen.
Given the very significant costs of powering satellites and space stations it is very desirable to extend the life of nickel-hydrogen cells as much as possible to reduce the cost of replacing the batteries in a space station or requiring the use of a new satellite when an old satellite could still function. Accordingly, there is a need in the art for a system to carefully control the re-charging of a nickel-hydrogen cell and any other electrical storage devices in a manner to mitigate damage during charging and extend the life of the storage device.